Field of the Disclosure
The present invention relates to the field of carbon dioxide mineralization, especially an apparatus and method for absorbing and mineralizing carbon dioxide.
Description of the Related Art
Collection and disposal of carbon dioxide (CO2), for reducing greenhouse gas emissions, is an important technical problem in the field of the environment and ecology. Currently, CO2 collection and disposal mainly includes geological storage, ocean storage, mineral carbonation and biological carbon sequestration. CO2 mineral carbonation sequestration refers to a series of processes in which CO2 reacts with minerals containing alkaline or alkaline-earth metal oxides (mainly calcium and magnesium silicate minerals) to produce carbonate thus to be stored. Mineral carbonation is the CO2 sequestration process in the nature, the resulting carbonates are thermodynamically stable form for carbon, and without any impact on the environment, and therefore mineral carbonation is a most stable and safest way of carbon sequestration; and, various minerals which can react with CO2 exist in the nature, with huge capacity and low prices; therefore, mineral carbonation carbon sequestration is one of the best approaches among CO2 collection and disposal technologies.
Currently, reported CO2 carbonation sequestration processes and technologies includes direct dry gas-solid carbonation and liquid phase absorption carbonation. Direct dry gas-solid carbonation employs the route in which CO2 react with minerals directly in one step gas-solid reaction to produce carbonate. This route is handicapped by low reaction rate and low efficiency, even performed under high pressure; it is difficult to meet the need of large-scale industrial absorption. Therefore, liquid phase absorption carbonation is considered to be the major route for mineral carbonation sequestration.
There are two different routes for liquid phase absorption carbonation, namely the direct and indirect absorption methods. Direct absorption method is as followed, after the calcium magnesium silicate (also the like minerals) is crushed into fine particles and dispersed into liquid phase, reacting with CO2 to produce carbonate. The chemical reaction of direct absorption method is the same with direct gas solid carbonation, just as in liquid phase, the CO2 is dissolved to form carbonate, while further reacting with fine mineral particles, by which the reaction rate has been improved. Cost of grinding minerals is very high, but the reaction rate still cannot meet the needs of large-scale absorption; therefore this method is still not the best choice in economy and efficiency.
In the indirect liquid phase absorption method, the minerals are first converted to alkaline solution or suspension (hereinafter referred to as alkali liquor), absorbing CO2 in the alkali liquor to form carbonates, and the carbonates are further separated to sequestration CO2. Two core steps of the method are conversion of the minerals and CO2 absorption. Depending on the minerals employed and different routes of converting to alkali liquor, different processes can be obtained; and according to different absorption-reaction systems (solutions, suspensions or emulsions), there are also different technical solutions for absorption and reaction. With regard to the technologies and characteristics of various solutions, they are discussed in details in the articles “Carbon capture and storage using alkaline industrial waste (Progress in Energy and Combustion Science, 38: 302-320) and “A review of mineral carbonation technology in sequestration of CO2 (Journal of Petroleum Science and Engineering, 109: 364-392).
Due to the widely existing and low price of sodium salts, as well as the high solubility of NaOH, the process of employing the sodium salts as absorbing mineral is firstly proposed and industrialized. A typical process is, for example, the process employing NaHCO3/Na2CO3 as carbonates as disclosed in PCT patent WO2009039445 (Chinese Patent CN101970084A). In addition, similar technology is the absorbing technology reported in PCT patent WO2010068924 (Chinese patent CN101896425A). Generally in these patents, by electrolyzing sodium salts (such as Na2SO4) to obtain NaOH and CO2 is reacted with NaOH to obtain NaHCO3 or Na2CO3. Patent WO2009039445 also discloses the design and the structure of bubble column employed in the absorption. But the technology is lack of competitiveness in economy because electrolysis to obtain NaOH is high energy consuming. However, since the solubility of NaOH in water is high, the absorption process is a neutralization reaction in solution, with a fast reaction rate, and the reactor design is also relatively simple.
The true meaning of indirect liquid absorption method lies in the use of calcium magnesium silicates with great reserves and low prices for the absorption of CO2, and carbonates such as CaCO3, MgCO3 and the like with solid state are finally produced. This method generally converts the calcium magnesium silicates to oxides and further produce alkaline, and then reacts with CO2 in a suspension or emulsion of the alkaline, generating carbonates. For example, PCT patent WO2013106730 (Chinese Patent CN104284707A) discloses an indirect route in which calcium silicates are employed, with HCl used as medium and magnesium salts used as OH-carrier. In this process, the absorption of CO2 is achieved by reacting with Ca(OH)2, Mg(OH)2, or a mixture of Mg(OH)2 and CaCl2 to produce carbonates. Besides, similar process is the process employed acetic acid as medium. The document “Characteristics of CO2 fixation by chemical conversion to carbonate salts” (Chemical Engineering Journal, 231: 287-293) reports the process in which CO2 is absorbed in ethanolamine (MEA), diethanolamine (DEA), or methyl diethanolamine (MDEA), in which CaCl2 is dissolved to generate carbonates. In these processes above mentioned, a core process is concerned that CO2 is dissolved in the liquid phase and reacted to produce carbonates. The dissolution rate of CO2, the dissolution rate of the alkaline chemicals and the precipitation and crystallization rate of carbonate will all affect absorption efficiency, and any of these steps is likely to be a control step in the absorption process, usually when this absorption reaction is carried out with a simple bubbling method in a bubble column, CO2 absorption and reaction rate is usually not high enough. For example, when absorption and conversion are carried out with an alkali metal hydroxide, since the solubility of this kind of hydroxides is very low, if absorbed with solution, the absorption capacity is very low, resulting in circulation of large amount of water at the same time, and therefore it is necessary to absorb CO2 with hydroxide suspension. Related kinetic studies show that the dissolution of hydroxide is the control step of suspension absorption process in which the kinetics characteristics is different from solution absorption. On the other hand, in conventional absorption reactions, absorption and separation are carried out in separate units, since the content of the carbonate is usually not high, which leads to transportation or circulation of large amount of water from the absorption unit to the separation unit, resulting in large energy consumption. So far, there is still no public report for this absorption process to specially develop absorption reactor integrated with reaction and separation with high efficiency.